A ZnO thin-film driven microcantilever for nanoscale actuation and sensing

Zinc oxide (ZnO) thin film as a piezoelectric material for microelectromechanical system (MEMS) actuators and sensors was evaluated. ZnO thin films were deposited using radio frequency (RF) magnetron sputtering. Process parameters such as gas ratio, working pressure, and RF power were optimized for crystalline structure. The ZnO thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Good quality of ZnO thin films was further confirmed by a high transverse piezoelectric coefficient d 31. A microcantilever was then designed, fabricated, and characterized. Design formulas of resonant frequency, actuation, and sensing sensitivities were derived. The resonant frequency was determined by an impedance analyzer. Tip deflection on nanometer level was demonstrated with the cantilever used as an actuator. The actuation sensitivity was found to be 12.2 nm/V. As a sensor, the cantilever was calibrated against a reference accelerometer. The sensing sensitivity was characterized to be 46 mV/g. The characterization results were compared with design specifications. The differences were caused mainly by thickness control in etching. This study showed that ZnO is a promising piezoelectric material for MEMS actuators and sensors in terms of excellent process compatibility and good piezoelectric performance.